Enantiomeric Isopulegol as the Chiral Pool in the Total Synthesis of Bioactive Agents

Isopulegol, a pool of abundant chiral terpene, has long served as the starting material for the total synthesis of isopulegol-based drugs. As an inexpensive and versatile starting material, this compound continues to serve modern synthetic chemistry. This review highlights the total syntheses of terpenoids in the period from 1980 to 2020 in which with isopulegol applied as a building block.

Influenza antiviral activity of F- and OH-containing isopulegol-derived octahydro-2H-chromenes

We synthesized fluoro- and hydroxy-containing octahydro-2H-chromenes by the Prins reaction starting from a monoterpenoid (-)-isopulegol and a wide range of aromatic aldehydes in the presence of the BF₃∙Et₂O/H₂O system acting as both an acid catalyst and a fluorine source. Activity of the produced compounds against the influenza A/Puerto Rico/8/34 (H1N1) virus was studied. The highest activity was demonstrated by fluoro- (11i) and hydroxy-containing (10i) derivatives of 2,4,6-trimethoxybenzaldehyde. The most pronounced virus-inhibiting effect of compounds 10i and 11i was observed at an early stage of infection. These compounds were supposed to be capable of binding to viral hemagglutinin, which is an agreement with data on the effect of compounds 10i and 11i on the viral fusogenic activity as well as by molecular docking studies.

Ene cyclization reaction in heterocycle synthesis

The ene cyclization has evolved to become an indispensable tool for the synthesis of various ring size heterocyclic compounds. In the past and recent years, many exciting reports have demonstrated the broad scope and synthetic utility of ene cyclization and the versatility of oxonium ion, iminium ion and thionium ion intermediates. Moreover, the ease of regio- and stereoselectivity of ene cyclization has led to the development of new types of heterocyclic compounds. This article aims at reviewing the utilities of ene cyclization reactions for the synthesis of various ring sizes of oxygen, nitrogen and sulfur heterocycles. It also covers some applications in natural product synthesis. The mechanism of the reactions is discussed wherever necessary. The review article covers the time period from 1986 to 2017.

Regio- and Diastereoselective Synthesis of Dihydropyrans and Pyranopyrans via Oxonium-Ene Reaction of β-Allenols and Aldehydes

Bismuth trifluoromethanesulfonate can be efficiently used for the preparation of dihydropyrans from β-allenols and aldehydes by oxonium-ene reaction in good yields. The reaction is highly regioselective. On the other hand, the same reaction with trimethylsilyl trifluoromethanesulfonate at -45 °C affords the hexahydropyrano[4,3- b]pyran skeleton in moderate yields.

Diastereoselective synthesis of tetrahydropyrans via Prins–Ritter and Prins–arylthiolation cyclization reactions

Highly diastereoselective Prins–Ritter and Prins-arylthiolation cyclization reactions have been developed for the synthesis of two novel classes of tetrahydropyran derivatives.

A practical way to synthesize chiral fluoro-containing polyhydro-2H-chromenes from monoterpenoids

Conditions enabling the single-step preparative synthesis of chiral 4-fluoropolyhydro-2H-chromenes in good yields through a reaction between monoterpenoid alcohols with para-menthane skeleton and aldehydes were developed for the first time. The BF₃·Et₂O/H₂O system is used both as a catalyst and as a fluorine source. The reaction can involve aliphatic aldehydes as well as aromatic aldehydes containing various acceptor and donor substituents. 4-Hydroxyhexahydro-2H-chromenes were demonstrated to be capable of converting to 4-fluorohexahydro-2H-chromenes under the developed conditions, the reaction occurs with inversion of configuration.

Synthesis of 4-trifluoromethanesulfonate substituted 3,6-dihydropyrans and their application in various C–C coupling reactions

Triflic acid is used for Prins cyclization of homopropargyl alcohols with aldehydes to give 3,6-dihydro-2H-pyran-4-yl trifluoromethanesulfonates. The dihydropyran is converted into 4-alkyl and aryl substituted products using cross-coupling reactions.